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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">virtual</span></span></span> 说明符指定非静态<a href="member_functions.html" title="cpp/language/member functions">成员函数</a>为<i>虚</i>函数并支持动态调用派发。它只能在非静态成员函数的首个声明（即当它于类定义中声明时）的 <span class="t-spar">声明说明符序列</span> 中出现。
</p>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p>虚函数是可在派生类中覆盖其行为的成员函数。与非虚函数相反，即使没有关于该类实际类型的编译时信息，仍然保留被覆盖的行为。当使用到基类的指针或引用来处理派生类时，对被覆盖的虚函数的调用，将会调用定义于派生类中的行为。当使用<a href="lookup.html" title="cpp/language/lookup">有限定名字查找</a>（即函数名出现在作用域解析运算符 <code>::</code> 的右侧）时，此行为被抑制。
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">struct</span> Base <span class="br0">{</span>
   <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
       <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"base<span class="es1">\n</span>"</span><span class="sy4">;</span>
   <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> Derived <span class="sy4">:</span> Base <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> override <span class="br0">{</span> <span class="co1">// 'override' 可选</span>
        <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"derived<span class="es1">\n</span>"</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    Base b<span class="sy4">;</span>
    Derived d<span class="sy4">;</span>
 
    <span class="co1">// 通过引用调用虚函数</span>
    Base<span class="sy3">&amp;</span> br <span class="sy1">=</span> b<span class="sy4">;</span> <span class="co1">// br 的类型是 Base&amp;</span>
    Base<span class="sy3">&amp;</span> dr <span class="sy1">=</span> d<span class="sy4">;</span> <span class="co1">// dr 的类型也是 Base&amp;</span>
    br.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印 "base"</span>
    dr.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印 "derived"</span>
 
    <span class="co1">// 通过指针调用虚函数</span>
    Base<span class="sy2">*</span> bp <span class="sy1">=</span> <span class="sy3">&amp;</span>b<span class="sy4">;</span> <span class="co1">// bp 的类型是 Base*</span>
    Base<span class="sy2">*</span> dp <span class="sy1">=</span> <span class="sy3">&amp;</span>d<span class="sy4">;</span> <span class="co1">// dp 的类型也是 Base*</span>
    bp<span class="sy2">-</span><span class="sy1">&gt;</span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印 "base"</span>
    dp<span class="sy2">-</span><span class="sy1">&gt;</span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印 "derived"</span>
 
    <span class="co1">// 非虚函数调用</span>
    br.<span class="me1">Base</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印 "base"</span>
    dr.<span class="me1">Base</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印 "base"</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<h3><span class="mw-headline" id=".E7.BB.86.E8.8A.82">细节</span></h3>
<p>若某个成员函数 <code>vf</code> 在类 <code>Base</code> 中被声明为 <code>virtual</code>，且某个直接或间接派生于 <code>Base</code> 的类 <code>Derived</code> 拥有一个下列几项与之相同的成员函数声明
</p>
<ul><li> 名字
</li><li> 形参列表（但非返回类型）
</li><li> cv 限定符
</li><li> 引用限定符
</li></ul>
<p>则类 <code>Derived</code> 中的此函数亦为<i>虚</i>函数（无论其声明中是否使用关键词 <code>virtual</code>）并<i>覆盖</i> Base::vf（无论其声明中是否使用单词 <code>override</code>）。
</p><p>要覆盖的 <code>Base::vf</code> 不需要可见（可声明为 private，或用私有继承继承）。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> B <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> do_f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 私有成员</span>
 <span class="kw1">public</span><span class="sy4">:</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> do_f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 公开接口</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> <span class="kw1">public</span> B <span class="br0">{</span>
    <span class="kw4">void</span> do_f<span class="br0">(</span><span class="br0">)</span> override<span class="sy4">;</span> <span class="co1">// 覆盖 B::do_f</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    D d<span class="sy4">;</span>
    B<span class="sy2">*</span> bp <span class="sy1">=</span> <span class="sy3">&amp;</span>d<span class="sy4">;</span>
    bp<span class="sy2">-</span><span class="sy1">&gt;</span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 内部调用 D::do_f();</span>
<span class="br0">}</span></pre></div></div>
<p>每个虚函数都有其<i>最终覆盖函数</i>，它是进行虚函数调用时所执行的函数。基类 <code>Base</code> 的虚成员函数 <code>vf</code> 是最终覆盖函数，除非派生类声明或（通过多重继承）继承了覆盖 <code>vf</code> 的另一个函数。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span> <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>     <span class="co1">// A::f 是虚函数</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>         <span class="co1">// B::f 覆盖 A::f in B</span>
<span class="kw1">struct</span> C <span class="sy4">:</span> <span class="kw1">virtual</span> B <span class="br0">{</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// C::f 覆盖 A::f in C</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> <span class="kw1">virtual</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// D 不引入覆盖函数，B::f 在 D 中为最终</span>
<span class="kw1">struct</span> E <span class="sy4">:</span> C, D  <span class="br0">{</span>       <span class="co1">// E 不引入覆盖函数，C::f 在 E 中为最终</span>
    <span class="kw1">using</span> A<span class="sy4">::</span><span class="me2">f</span><span class="sy4">;</span> <span class="co1">// 非函数声明，仅令 A::f 能为查找所见</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
   E e<span class="sy4">;</span>
   e.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// 虚调用调用 C::f，e 中的最终覆盖函数</span>
   e.<span class="me1">E</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 非虚调用调用 A::f，它在 E 中可见</span>
<span class="br0">}</span></pre></div></div>
<p>若一个函数拥有多于一个最终覆盖函数，则程序非良构：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> VB1 <span class="sy4">:</span> <span class="kw1">virtual</span> A <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 覆盖 A::f</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> VB2 <span class="sy4">:</span> <span class="kw1">virtual</span> A <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 覆盖 A::f</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// struct Error : VB1, VB2 {</span>
<span class="co1">//     // 错误：A::f 在 Error 中拥有两个最终覆盖函数</span>
<span class="co1">// };</span>
<span class="kw1">struct</span> Okay <span class="sy4">:</span> VB1, VB2 <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：这是 A::f 的最终覆盖函数</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> VB1a <span class="sy4">:</span> <span class="kw1">virtual</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 不声明覆盖函数</span>
<span class="kw1">struct</span> Da <span class="sy4">:</span> VB1a, VB2 <span class="br0">{</span>
    <span class="co1">// Da 中，A::f 的最终覆盖函数是 VB2::f</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>具有相同名字但不同形参列表的函数并不覆盖同名的基类函数，但会<i>隐藏</i>它：在<a href="lookup.html" title="cpp/language/lookup">无限定名字查找</a>检查派生类的作用域时，查找找到该声明而不再检查基类。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> B <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// D::f 隐藏 B::f（错误的形参列表）</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D2 <span class="sy4">:</span> D <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// D2::f 覆盖 B::f（它不可见也不要紧）</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    B b<span class="sy4">;</span>   B<span class="sy3">&amp;</span> b_as_b   <span class="sy1">=</span> b<span class="sy4">;</span>
    D d<span class="sy4">;</span>   B<span class="sy3">&amp;</span> d_as_b   <span class="sy1">=</span> d<span class="sy4">;</span>    D<span class="sy3">&amp;</span> d_as_d <span class="sy1">=</span> d<span class="sy4">;</span>
    D2 d2<span class="sy4">;</span> B<span class="sy3">&amp;</span> d2_as_b  <span class="sy1">=</span> d2<span class="sy4">;</span>   D<span class="sy3">&amp;</span> d2_as_d <span class="sy1">=</span> d2<span class="sy4">;</span>
 
    b_as_b.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 B::f()</span>
    d_as_b.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 B::f()</span>
    d2_as_b.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 D2::f()</span>
 
    d_as_d.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：D 中的查找只找到 f(int)</span>
    d2_as_d.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：D 中的查找只找到 f(int)</span>
<span class="br0">}</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>若函数以说明符 <code>override</code> 声明，但不覆盖任何虚函数，则程序非良构：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> B <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> override<span class="sy4">;</span> <span class="co1">// OK，D::f(int) 覆盖 B::f(int)</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">long</span><span class="br0">)</span> override<span class="sy4">;</span> <span class="co1">// 错误：f(long) 不覆盖 B::f(int)</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>若函数以说明符 <code>final</code> 声明，而另一函数试图覆盖之，则程序非良构：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> final<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> B <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span> <span class="co1">// 错误：D::f 试图覆盖 final B::f</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<p>非成员函数和静态成员函数不能为虚函数。
</p><p>函数模板不能被声明为 <code>virtual</code>。这只适用于自身是模板的函数——类模板的常规成员函数可被声明为虚函数。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<p>虚函数（无论是声明为 virtual 者还是覆盖函数）不能有任何关联制约
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> requires <span class="kw2">true</span><span class="sy4">;</span> <span class="co1">// 错误：受制约的虚函数</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p><a href="consteval.html" title="cpp/language/consteval"><code>consteval</code></a> 虚函数不能覆盖非 <code>consteval</code> 虚函数或被它覆盖。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td></tr>
</table>
<p>在编译时替换虚函数的<a href="default_arguments.html" title="cpp/language/default arguments">默认实参</a>。
</p>
<h4><span class="mw-headline" id=".E5.8D.8F.E5.8F.98.E8.BF.94.E5.9B.9E.E7.B1.BB.E5.9E.8B">协变返回类型</span></h4>
<p>若函数 <code>Derived::f</code> 覆盖 <code>Base::f</code>，则其返回类型必须要么相同要么为<i>协变（covariant）</i>。当满足所有下列要求时，两个类型为协变：
</p>
<ul><li> 两个类型均为到类的指针或引用（左值或右值）。不允许多级指针或引用。
</li><li> <code>Base::f()</code> 的返回类型中被引用/指向的类，必须是 <code>Derived::f()</code> 的返回类型中被引用/指向的类的无歧义且可访问的直接或间接基类。
</li><li> <code>Derived::f()</code> 的返回类型必须有相对于 <code>Base::f()</code> 的返回类型的相等或较少的 <a href="cv.html" title="cpp/language/cv">cv 限定</a>。
</li></ul>
<p><code>Derived::f</code> 的返回类型中的类必须要么是 <code>Derived</code> 自身，要么必须是于 <code>Derived::f</code> 声明点的某个<a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">完整类型</a>。
</p><p>进行虚函数调用时，最终覆盖函数的返回类型被<a href="implicit_cast.html" title="cpp/language/implicit conversion">隐式转换</a>成所调用的被覆盖函数的返回类型：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Base <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> vf1<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> vf2<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> vf3<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw1">virtual</span> B<span class="sy2">*</span> vf4<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw1">virtual</span> B<span class="sy2">*</span> vf5<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">class</span> D <span class="sy4">:</span> <span class="kw1">private</span> B <span class="br0">{</span>
    <span class="kw1">friend</span> <span class="kw1">struct</span> Derived<span class="sy4">;</span> <span class="co1">// Derived 中，B 是 D 的可访问基类</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">class</span> A<span class="sy4">;</span> <span class="co1">// 前置声明的类是不完整类型</span>
 
<span class="kw1">struct</span> Derived <span class="sy4">:</span> <span class="kw1">public</span> Base <span class="br0">{</span>
    <span class="kw4">void</span> vf1<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// 虚函数，覆盖 Base::vf1()</span>
    <span class="kw4">void</span> vf2<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 非虚函数，隐藏 Base::vf2()</span>
<span class="co1">//  char vf3();    // 错误：覆盖 Base::vf3，但具有不同且非协变的返回类型</span>
    D<span class="sy2">*</span> vf4<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>      <span class="co1">// 覆盖 Base::vf4() 并具有协变的返回类型</span>
<span class="co1">//  A* vf5();      // 错误：A 是不完整类型</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    Derived d<span class="sy4">;</span>
    Base<span class="sy3">&amp;</span> br <span class="sy1">=</span> d<span class="sy4">;</span>
    Derived<span class="sy3">&amp;</span> dr <span class="sy1">=</span> d<span class="sy4">;</span>
 
    br.<span class="me1">vf1</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 Derived::vf1()</span>
    br.<span class="me1">vf2</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 Base::vf2()</span>
<span class="co1">//  dr.vf2(); // 错误：vf2(int) 隐藏 vf2()</span>
 
    B<span class="sy2">*</span> p <span class="sy1">=</span> br.<span class="me1">vf4</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 Derived::vf4() 并将结果转换为 B*</span>
    D<span class="sy2">*</span> q <span class="sy1">=</span> dr.<span class="me1">vf4</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 Derived::vf4() 而不将结果转换为 B*</span>
 
<span class="br0">}</span></pre></div></div>
<h4><span class="mw-headline" id=".E8.99.9A.E6.9E.90.E6.9E.84.E5.87.BD.E6.95.B0">虚析构函数</span></h4>
<p>虽然析构函数是不继承的，但若基类声明其析构函数为 <code>virtual</code>，则派生的析构函数始终覆盖它。这使得可以通过指向基类的指针 delete 动态分配的多态类型对象
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Base <span class="br0">{</span>
 <span class="kw1">public</span><span class="sy4">:</span>
    <span class="kw1">virtual</span> ~Base<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/* 释放 Base 的资源 */</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">class</span> Derived <span class="sy4">:</span> <span class="kw1">public</span> Base <span class="br0">{</span>
    ~Derived<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/* 释放 Derived 的资源 */</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    Base<span class="sy2">*</span> b <span class="sy1">=</span> new Derived<span class="sy4">;</span>
    delete b<span class="sy4">;</span> <span class="co1">// 进行对 Base::~Base() 的虚函数调用</span>
              <span class="co1">// 由于它是虚函数，故它调用的是 Derived::~Derived()，</span>
              <span class="co1">// 这就能释放派生类的资源，然后遵循通常的析构顺序</span>
              <span class="co1">// 调用 Base::~Base()</span>
<span class="br0">}</span></pre></div></div>
<p>此外，若类是<i>多态的</i>（声明或继承了至少一个虚函数），且其析构函数非虚，则删除它是<i>未定义行为</i>，无论不调用派生的析构函数时是否会导致资源泄漏。
</p><p>一条有用的方针是，任何基类的析构函数必须为<a rel="nofollow" class="external text" href="https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#discussion-make-base-class-destructors-public-and-virtual-or-protected-and-nonvirtual">公开且虚，或受保护且非虚</a>。
</p>
<h3><span class="mw-headline" id=".E5.9C.A8.E6.9E.84.E9.80.A0.E5.92.8C.E6.9E.90.E6.9E.84.E6.9C.9F.E9.97.B4">在构造和析构期间</span></h3>
<p>当从构造函数或从析构函数中直接或间接调用虚函数（包括在类的非静态数据成员的构造或析构期间，例如在成员<a href="constructor.html" title="cpp/language/initializer list">初始化器列表</a>中），且对其实施调用的对象是正在构造或析构中的对象时，所调用的函数是构造函数或析构函数的类中的最终覆盖函数，而非进一步的派生类中的覆盖函数。 
换言之，在构造和析构期间，进一步的派生类并不存在。
</p><p>当构建具有多个分支的复杂类时，在属于一个分支的构造函数内，多态被限制到该类及其基类：若它获得了指向这个子层级之外的某个基类子对象的指针或引用，且试图进行虚函数调用（例如通过显式成员访问），则行为未定义：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> V <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> A <span class="sy4">:</span> <span class="kw1">virtual</span> V <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// A::f 是 V::f 在 A 中的最终覆盖函数</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> <span class="kw1">virtual</span> V <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// B::g 是 V::g 在 B 中的最终覆盖函数</span>
    B<span class="br0">(</span>V<span class="sy2">*</span>, A<span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> A, B <span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// D::f 是 V::f 在 D 中的最终覆盖函数</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// D::g 是 V::g 在 D 中的最终覆盖函数</span>
 
    <span class="co1">// 注意：A 在 B 之前初始化</span>
    D<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> B<span class="br0">(</span><span class="br0">(</span>A<span class="sy2">*</span><span class="br0">)</span>this, this<span class="br0">)</span> 
    <span class="br0">{</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// B 的构造函数，从 D 的构造函数调用 </span>
B<span class="sy4">::</span><span class="me2">B</span><span class="br0">(</span>V<span class="sy2">*</span> v, A<span class="sy2">*</span> a<span class="br0">)</span>
<span class="br0">{</span>
    f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 对 V::f 的虚调用（尽管 D 拥有最终覆盖函数，D 也不存在）</span>
    g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 对 B::g 的虚调用，在 B 中是最终覆盖函数</span>
 
    v<span class="sy2">-</span><span class="sy1">&gt;</span>g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// v 的类型 V 是 B 的基类，虚调用如前调用 B::g</span>
 
    a<span class="sy2">-</span><span class="sy1">&gt;</span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// a 的类型 A 不是 B 的基类，它属于层级中的不同分支。</span>
            <span class="co1">// 尝试通过这个分支进行虚调用导致未定义行为，</span>
            <span class="co1">// 即使此情况下 A 已完成构造</span>
            <span class="co1">// （它在 B 之前构造，因为它在 D 的基类列表中先于 B 出现）</span>
            <span class="co1">// 实践中，对 A::f 的虚调用会试图使用 B 的虚成员函数表，</span>
            <span class="co1">// 因为它在 B 的构造期间是活跃的</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<ul><li> <a href="derived_class.html" title="cpp/language/derived class">派生类和继承模式</a>
</li><li> <a href="override.html" title="cpp/language/override">override 说明符</a> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</li><li> <a href="final.html" title="cpp/language/final">final 说明符</a> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</li></ul>

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